1. Field of the Invention
The present invention relates to a vapor deposition method and, more particularly, to a vapor deposition method for forming a thin film having uniform distribution of thickness on a substrate.
2. Description of the Prior Art
Conventionally, a vapor deposition method for forming an electroluminescent (EL) thin film is sometimes implemented in an apparatus shown in FIG. 10. In this apparatus, an evaporation source 11 comprising a deposition material such as zinc sulfide (ZnS), whose deposition rate depends on a temperature of a substrate, is opposed to the substrate 12 of an EL element in a vacuum chamber. In this vacuum chamber provided are heaters 14 and 16 apart from the substrate 12. In addition, a reflecting plate 13 is provided close to the substrate on the side opposite to the evaporation source 11. The substrate 12 is heated up by the heaters 14 and 16 and the reflecting plate 13 adjusts a quantity of heat transmitted from the heaters 14 and 16 to the substrate 12 by reflecting infrared rays. Then, the deposition material of the evaporation source 11 is deposited on the substrate 12 while a temperature of the substrate 12 is uniformly kept at approximately 250.degree. C. As shown in FIG. 9, when the substrate temperature is Ts to 250.degree. C., every time the substrate temperature Ts changes by 1.degree. C., the EL thin film thickness changes by approximately 1%. A material of the reflecting plate 13 is stainless steel SS, so that it does not react with the deposition material. In addition, since the reflecting plate 13 has a specular surface, the infrared rays from the heaters 14 and 16 are effectively reflected thereon.
Referring to the EL thin film formed by the above-described vapor deposition method, uniformity of the film thickness is sufficient for a thin film EL element with non-gradation display (monochrome display) but it is not sufficient for a thin film EL element with 16-gradation display. The reason for this is that allowable distribution of the film thickness on the substrate when 16-gradation display function is implemented is .+-.2% and further it is preferably .+-.1% in order to improve display grade while it is approximately .+-.5% in a case of the thin film EL element with non-gradation display. In addition, in the 16-gradient in which non-luminous state is at B0 level and brightest luminous state is at B15 level, the above .+-.2% is determined so that a luminance distribution at B1 level (referred to as B1 level luminance distribution hereinafter) of a darkest luminous state may be 40% or more. As described above, according to the conventional vapor deposition method, the EL thin film can not satisfy that level of the 16-gradation display and uniformity of the film thickness is not sufficient.
It is an object of the present invention to provide a vapor deposition method in which uniformity of thickness of a thin film to be formed can be improved and the level of the 16-gradation display can be satisfied when the EL thin film is formed.